signals: label plots by row and cell, drop onto a cell

src/ui_probe.cpp

@@ -4259,8 +4259,73 @@ gboolean split_probe_step(gpointer user) {
                    cell_plus_sensitive, row_plus_sensitive, rs.row_count());
       // Close editor.
       gtk_toggle_button_set_active(GTK_TOGGLE_BUTTON(a->layout_toggle), false);
+      split_probe_next(*a);
       break;
     }
+    // --- Steps 16..17: signals page and drop onto a cell (task 6) ---
+    case 16: {
+      // Two split rows (plots 0,1 on row 1; plots 2,3 on row 2), all empty; the next step reads a frame
+      // that was laid out with them.
+      rs.set_cells_per_row({2, 2});
+      split_probe_clear_all(*a);
+      split_probe_restructured(*a);
+      split_probe_next(*a);
+      break;
+    }
+    case 17: {
+      split_probe_report(*a, "two-split-rows");
+      // Which plot the pointer resolves to at the centre of every cell, through the app's own wrapper
+      // (`row_at(app, x, y)`), in the area's logical coordinates: the cell rects are device pixels.
+      const plot::FigureLayout& fl = rs.figure().layout();
+      const double scale = (a->area != nullptr && a->area->device_scale() > 0.0f)
+                               ? static_cast<double>(a->area->device_scale()) : 1.0;
+      std::string rowat;
+      int rowat_ok = fl.cells.size() == static_cast<std::size_t>(rs.row_count()) ? 1 : 0;
+      for (std::size_t i = 0; i < fl.cells.size() && rowat_ok; ++i) {
+        const plot::Rect& c = fl.cells[i];
+        const int got = row_at(*a, (static_cast<double>(c.x) + c.w * 0.5) / scale,
+                               (static_cast<double>(c.y) + c.h * 0.5) / scale);
+        rowat += (i == 0 ? "" : ",") + rs.plot_label(static_cast<int>(i)) + "=" + std::to_string(got);
+        if (got != static_cast<int>(i)) rowat_ok = 0;
+      }
+      std::fprintf(stderr, "[split-probe] drop-rowat      ok=%d rowat=[%s] (label=plot index at each cell's centre)\n",
+                   rowat_ok, rowat.c_str());
+      // The signals page's "Plot on" buttons for signal 1: label and tooltip of each, read off the widgets.
+      const std::string sig0 = a->model->signals()[1].name;
+      signal_page_ensure(*a, sig0);
+      signal_pages_sync(*a);
+      std::string buttons;
+      const auto page_it = a->signal_pages.find(sig0);
+      GtkWidget* box = page_it == a->signal_pages.end() ? nullptr
+                                                        : find_named(page_it->second, "udpscope-plot-on");
+      for (GtkWidget* b = box != nullptr ? gtk_widget_get_first_child(box) : nullptr; b != nullptr;
+           b = gtk_widget_get_next_sibling(b)) {
+        const char* tip = gtk_widget_get_tooltip_text(b);
+        buttons += std::string(buttons.empty() ? "" : "|") + gtk_button_get_label(GTK_BUTTON(b)) + ":" +
+                   (tip != nullptr ? tip : "-");
+      }
+      std::fprintf(stderr, "[split-probe] plot-on-buttons [%s]\n", buttons.c_str());
+      // The "Plot on 1b" path: the button's own function, onto plot 1 (the right cell of row 1).
+      const bool assigned = assign_signal_to_row(*a, 1, 1);
+      std::fprintf(stderr, "[split-probe] assign-1b       assigned=%d notice='%s'\n", assigned ? 1 : 0,
+                   a->notice.c_str());
+      // And the drop handler itself, at the centre of cell 3 ("2b"), with signal 2's index as the payload.
+      GValue payload = G_VALUE_INIT;
+      g_value_init(&payload, G_TYPE_STRING);
+      g_value_set_string(&payload, "2");
+      const plot::Rect& c3 = fl.cells[3];
+      const gboolean dropped = on_drop_channel(nullptr, &payload, (static_cast<double>(c3.x) + c3.w * 0.5) / scale,
+                                               (static_cast<double>(c3.y) + c3.h * 0.5) / scale, a);
+      g_value_unset(&payload);
+      std::fprintf(stderr, "[split-probe] drop-2b         dropped=%d notice='%s'\n", dropped ? 1 : 0,
+                   a->notice.c_str());
+      split_probe_restructured(*a);
+      split_probe_next(*a);
+      break;
+    }
+    case 18:
+      split_probe_report(*a, "after-assign");
+      break;
     default:
       break;
   }